1. Field of the Invention
The invention relates to circuits for charging of a battery and particularly a battery of an emergency lighting system, the circuits providing a low impedance alternate current path for operation of lamps in a non-emergency mode once battery charging is complete.
2. Description of the Prior Art
Conventional emergency lighting systems which utilize one or more batteries for DC operation (or chopped AC or other AC derived from DC battery operation) require charging of the batteries with AC power derived from conventional AC mains. Battery charging systems used in emergency lighting systems and in other systems vary greatly in capabilities depending upon the requirements of a particular charging application. In most typical applications, battery chargers are designed to charge with a substantially constantly applied voltage. Such chargers use a relatively "stiff" voltage source such as a power transformer in series with a bridge rectifier. Charging circuits of this constant voltage type can allow damage to the voltage source in the event that battery leads are shorted or if the circuit is connected to a severely damaged battery inter alia, an inherent current limiting capability or series impedance being necessary in such a circuit to prevent voltage source damage. However, placement of a series impedance, for example, in a constant voltage battery charging circuit will cause tapering of the charge current as the voltage of the battery increases, thereby causing less voltage to be present across the series impedance. The practice of providing such a series impedance is actually counterproductive since the voltage source must be designed to handle maximum charging currents and to achieve maximum capacity within a reasonably short time. For these reasons, constant voltage charging systems are typically designed to maintain a high charging rate until the battery is fully charged. In the systems designed for maintenance of a high current rate, a short circuit or damaged battery condition creates a highly dissipative condition in which the source voltage drops across the series impedance. The use of current limited transformers designed to alleviate these problems still results in a sufficiently dissipative condition as to create a severe design limitation. For the reasons thus provided, prior art battery charging systems commonly employ large heat sinks on series pass elements and even incorporate voltage regulators having thermal overload protection. Even under optimum conditions, the voltage regulators drop three to four volts across the terminals of the regulators at the desired charge current level, thereby resulting in losses (heat) which must ultimately be dissipated within the confines of the charger housing. Certain types of batteries further require more than one charge level. Batteries may also require charge termination or charge control such as a "trickle" charge. In such situations, the initial charge current may necessarily be much higher than the charge required to maintain a fully charged state. Once a battery is fully charged at the high initial level, it becomes necessary to drastically reduce or terminate the charge current in order to prevent battery degradation.
Prior battery charging systems are common in the art and include circuits such as the battery charging circuit of Bulat described in U.S. Pat. No. 4,223,232 which utilizes a single active element to regulate DC voltage levels from an AC source for battery charging and to power an emergency system upon loss of the AC source. The Bulat circuit also functions to disconnect the battery from the emergency system to prevent deep discharge and resultant battery damage. Feldstein, in U.S. Pat. Nos. 4,216,410 and 4,454,452, charges a battery of an emergency lighting system by means of a rectifier charging circuit operating in series with lamp current, the battery charging current being limited. On failure of AC power, the lamp is isolated from the AC line and operated by an inverter drawing power from the battery. In U. S. Pat. No. 4,890,004, Beckerman describes a battery charging system utilizing a temperature compensated switched voltage regulator. Folts in U.S. Pat. No. 5,302,858 charges a battery in a backup power system when power is available from the main AC power system through utilization of the main power transformer and the main inverter.
The prior art as represented by the aforesaid patents and by the battery charging circuits and systems generally described exhibit serious deficiencies in function and maintenance for which the industry has experienced a long-felt need for remedy thereof. Such remedy is provided by the battery charging circuits of the present invention which minimize circuit losses and thereby eliminate the need for large heat sinks or thermally protected devices. The reduction in losses (heat) thereby act to improve component life by allowing the circuit components to operate at reduced ambient temperatures. This ability to allow operation at reduced temperatures acts in some situations to reduce or eliminate product discolorations which can occur due to long term heating affects experienced in emergency products such as exit signs and the like. The present circuits also provide a battery charging system capable of maintaining a high charge level until the battery is fully charged and to then shunt the current from the charge path to a low impedance path for operation of non-emergency system lighting while retaining the ability to effect continual charge control.